This invention relates to a process for purifying a polycarboxylic acid. More specifically, the invention relates to a process for purifying a polycarboxylic acid made by the biological oxidation of a substrate by a microorganism from a fermentation broth.
Aliphatic dioic acids are versatile chemical intermediates useful as raw materials for the preparation of perfumes, polymers, adhesives and macrolid antibiotics. The purity of dicarboxylic acids is critical when they are used to make polyesters because of the dependency of the molecular weight of the polyester on the purity of the dicarboxylic acid. It is well known that maximum molecular weight of a polymer produced by polycondensation will be attained under conditions favoring the most efficient reaction of the functional groups. Such conditions are attained when undesirable side reactions are substantially eliminated through, inter alia, the use of high purity bifunctional reagents. While several chemical routes to the synthesis of long-chain alpha, omega dicarboxylic acids are available, the synthesis is not easy and most methods result in mixtures containing shorter chain lengths. As a result, extensive purification steps are necessary. Dicarboxylic acids, especially long-chain dicarboxylic acids can also be produced by microbial transformation of alkanes, fatty acids or esters thereof. Such syntheses result in dicarboxylic acids having monocarboxylic acid impurities. The presence of monocarboxylic acids can substantially affect the molecular weight of any polycondensation polymer made with dicarboxylic acids having monocarboxylic acid impurities. There is, therefore, a need for methods for preparing highly pure dicarboxylic acids.
The present invention relates to a process for purifying a polycarboxylic acid. The method comprises contacting a polycarboxylic acid with a solvent to produce a substantially pure polycarboxylic acid. The solvent is chosen so that the impurities are soluble in the solvent and the polycarboxylic acid is insoluble in the solvent.
Not applicable.
Except in the claims and the operating examples, or where otherwise expressly indicated, all numerical quantities in this description indicating amounts of material or conditions of reaction and/or use are to be understood as modified by the word xe2x80x9caboutxe2x80x9d in describing the broadest scope of the invention.
It is understood that a carboxylic acid is any compound containing one or more carboxyl groups. A polycarboxylic acid is any compound having two or more carboxyl groups.
The present invention relates to a process for purifying a polycarboxylic acid. The method comprises contacting a polycarboxylic acid with a hydrocarbon solvent to produce a substantially pure polycarboxylic acid. The solvent is chosen so that the impurities are soluble in the hydrocarbon solvent and the polycarboxylic acid is insoluble in the solvent. Substantially pure polycarboxylic acids are substantially free of detectable amounts of monocarboxylic acids, mono- and dicarboxylic acid esters, nitrogen-containing materials and, sulfur-containing materials. Typically, substantially pure polycarboxylic acids produced by the process according to the invention contain less than about 2% by weight of detectable amounts of monocarboxylic acids, mono- and dicarboxylic acid esters, nitrogen-containing materials and, sulfur-containing materials.
The process according to the invention can be used to purify any polycarboxylic acid, especially those polycarboxylic acids having monocarboxylic acid impurities as the major impurities. The polycarboxylic acids that can be purified by process according to the invention include aliphatic dicarboxylic acids such as oxalic, malonic, adipic, azelaic, sebacic, dodecanedioc, 1,18-octadecendioic acid and the like; low molecular weight aliphatic tricarboxylic acids such as citric acid; aromatic dicarboxylic acids such as phthalic, isophthalic and terephthalic acids; aromatic dicarboxylic acids such as, trimellitic acid.
The solvent that can be used in the process according to the invention is any aliphatic or aromatic hydrocarbon solvent and/or mixtures thereof in which the monocarboxylic acid impurities present in the crude dicarboxylic acid are soluble and in which the dicarboxylic acid is substantially insoluble. Examples of such aliphatic solvents include, but are not limited to, linear and branched, cyclic and acyclic alkanes such as pentane, hexane, heptane, octane, 2,2,4-trimethylpentane, cyclopentane, cyclohexane, methylcyclopentane, methylcyclohexane; alkenes such as pentene, hexene, heptene, cyclopentene, cyclohexene, methylcyclopentene, methylcyclohexene and the like and liquefied hydrocarbons that are normally gases at room temperature and pressure such as liquid propane and liquid butane. Examples of such aromatic solvents include, but are not limited to, benzene, toluene, xylene. Solvent mixtures include, but are not limited to, petroleum distillates such as naphtha and petroleum ether.
The crude polycarboxylic acid is contacted by the hydrocarbon solvent for an effective time and in an effective amount sufficient to remove substantially all detectable amounts of monocarboxylic acids, mono- and dicarboxylic acid esters, nitrogen-containing materials and, sulfur-containing materials. The contacting can be carried out in any manner that is effective in removing at least the above impurities such as by stirring a slurry of solid crude polycarboxylic acid, passing the hydrocarbon solvent through a zone of solid crude polycarboxylic acid such as a column or tower of solid crude polycarboxylic acid. Liquid crude polycarboxylic acids can be contacted by heating a solid crude polycarboxylic acid and the hydrocarbon solvent until the polycarboxylic acid melts thereby forming two liquid phases which are then intimately contacted such as by stirring. The method of contacting the solid crude polycarboxylic acid with the hydrocarbon solvent and the time required will depend upon the nature of the polycarboxylic acid, the hydrocarbon solvent and the amount and type of impurities in the solid crude polycarboxylic acid and will be readily determinable by those of ordinary skill in the art.
The process according to the invention is particularly useful for purifying long chain aliphatic dicarboxylic acids having long chain monocarboxylic acid impurities. The process is especially useful for the purification of long chain aliphatic dicarboxylic acids produced by fermentation such as by the methods described in U.S. Pat. Nos. 5,254,466 and 5,620,878, the entire contents of each of which are incorporated herein by reference. A particularly preferred embodiment of the process according to the invention is the purification of mixed saturated and unsaturated dicarboxylic acids having primarily oleic acid impurities by contacting the long chain dicarboxylic acids with a hydrocarbon solvent.